add deprecation days support for beta and stable levels

[tcdsv]

closes #283

• the changelog and breaking command now include deprecation-days flags for both beta and stable versions.
• updated ChangelogFlags with new deprecation days flags
• removed hardcoded deprecation days values
• removed unused DeprecationDays field from ChangelogFlags and diff.Config

[reuvenharrison]

Looks great.
Please update API-DEPRECATION.md

[codecov-commenter]

Codecov Report

Merging #316 (a1f07ca) into main (8deecbf) will not change coverage.
The diff coverage is 81.81%.

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@@           Coverage Diff           @@
##             main     #316   +/-   ##
=======================================
  Coverage   77.84%   77.84%           
=======================================
  Files         177      177           
  Lines        8918     8918           
=======================================
  Hits         6942     6942           
  Misses       1701     1701           
  Partials      275      275           

Flag	Coverage Δ
unittests	77.84% <81.81%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
checker/deprecation.go	75.00% <ø> (ø)
diff/config.go	100.00% <ø> (ø)
internal/changelog_flags.go	100.00% <ø> (ø)
checker/default_checks.go	90.56% <66.66%> (ø)
internal/breaking_changes.go	100.00% <100.00%> (ø)
internal/changelog.go	70.40% <100.00%> (ø)

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